There are a variety of kinds of alarm devices such as fire alarm devices, gas alarm devices, burglar alarm devices, and security alarm devices. In these alarm devices, in accordance with an abnormal situation related to an alarm, a physical quantity such as the amount of light or temperature is measured selectively, and a variety of sensors are used for an alarm device in accordance with the usage.
For example, there are a variety of systems of a fire alarm device, that is a fire alarm, as a fire detection system such as a smoke detection system and a heat detection system. A typical smoke-detection fire alarm is a photoelectric one utilizing light scattering caused by smoke.
A photoelectric smoke detector has a structure in which a light-emitting element which emits light having high directivity in a circular container and a light-receiving element such as a photodiode are provided (for example, see Patent Document 1). In addition, a circuit board which controls the light-emitting element and the light-receiving element, performs arithmetic processing of measurement data, and transmits an electric signal is provided in a space isolated from a space in which smoke in the container is detected.
In the fire alarm, the light-emitting element and the light-receiving element are arranged so that light emitted from the light-emitting element does not enter the light-receiving element when no fire exists in the container. However, when smoke exists in the container, light emitted from the light-emitting element can be scattered by smoke and enter the light-receiving element. Light from the light-emitting element enters the light-receiving element, whereby the fire alarm detects a fire.